When manufacturing aircraft structures, such as for example wings, it is necessary to join one aircraft component to another. It is often the case that aircraft components are manufactured in such a way that when joining two components a shim is needed to fill a gap between the components. The use of a shim when joining components together is common, for example, when joining composite material aircraft structures such as spars and wing skins, owing to difficulties in manufacturing large composite parts accurately to a pre-defined shape. Generally, after a composite component has been moulded to a particular shape, the component changes shape due to effects such as the shrinkage of the resin matrix and other material effects in view of the temperatures of different regions of the material in the mould and in view of the different fibre lay-up directions employed. Such a change in shape is generally referred to as “spring back”. Predicting accurately how much a component will spring back after moulding is difficult, if not impossible. It is therefore difficult for component manufacturers to assemble composite components to the tight tolerances required for efficient aircraft production and operation. It may therefore be accepted that such components will spring back, and that the components to be joined together will not, without employing further means, fit together exactly (or close enough to be within acceptable tolerances). Thus, such components may be manufactured such that there will be a gap, albeit small, between adjacent components that is filled during assembly by means of a shim.
Shims have, to date, been produced by various techniques including machining a solid shim from a separate piece of metal material (such as a piece of aluminium). Solid shims are machined and fettled to fit in the gap and then sealant is used to fix the shim in the gap. Such machining/fettling is time consuming and requires a skilled operator.
Shims have also been produced with the use of sealant without employing any metal pieces. Forming a shim with sealant may be effected by coating either or both surfaces to be joined with sealant, then bringing the surfaces together so that the sealant is squeezed between the components and out from between the components, thereby filling the gap with sealant. Excess sealant squeezed out of the gap is then wiped away. Joining one component to the other using sealant in this way can lead to voids being present between the two components, where insufficient sealant has been provided. Also, filling gaps with sealant in such a way can be inefficient and can even reduce the effectiveness of the join between the two components.